1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly, to a single transverse-mode semiconductor laser device or laser diode (LD) having an optical waveguide structure providing high optical output characteristics.
2. Description of the Related Art
In today's world, optoelectronic technologies have achieved a remarkable development and have found a variety of applications including those of recording/reproducing information such as compact disks (CDs) and those of optical communication that uses optical fibers. Various LDs have beeen developed in the course of the development of optoelectronic technologies. For instance, semiconductor laser diodes adapted to near infrared or visible light are used for CDs, whereas those adapted to a large wavelength band are used for optical communication. Thus, LDs take a significant part in the optoelectronic technology.
Among LDs that are currently available and structurally differentiated, waveguide type LDs typically have a configuration adapted to a single-transverse-mode waveguiding. CDs are required to have a high recording density in order to store a large volume of data and a single-transverse-mode laser beam has to be used for recording data densely. Moreover, for optical communication, the use of a multi-mode signal beam is not suitable for long distance transmission because of the adverse effect of multi-mode dispersion so that waveguide LDs adapted to emit a single-transverse-mode laser beam are generally used.
In order to produce a single-transverse-mode beam, a single mode waveguide that has a limited width and is adapted to cut off any multi-mode beam is typically used for the waveguide of an LD. More specifically, the width of the active layer of the waveguide of the LD is limited to about 2 to 4 .mu.m. Thus, only a small electric current can be injected into the LD which results in limiting the optical output of the LD. One of the simplest ways to inject a large electric current and improve the saturated optical output is to use a waveguide having a large width for the LD. However, this technique contradicts the restrictions for realizing a single-transverse-mode waveguide as described above, so that the output level of an LD has so far been limited for technical reasons. Various solutions have been proposed to solve this problem.
A first prior art LD uses a wide mode-filter integrated multi-mode waveguide to form a major optical excitaion region to improve the saturated optical output. The multi-mode waveguide excites not only a single mode beam but also higher order mode beams such as a first-order mode beam and a second-order mode beam; in this case, the higher order mode beams are extracted by the mode filter (see P. Vankwikelberge et al., "Local Normal Mode Analysis of Index-Guided AlGaAs Lasers with Mode Filter", IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, pp. 730-737, June 1987).
However, since neither the optical energy of the first-order mode beam nor the optical energy of the second-order mode beam contribute to the optical output of the single mode beam, this first prior art LD shows a poor efficiency of transforming electric energy into optical energy if compared with the conventional single mode LD.
A second prior art LD uses a phase-locked semiconductor laser diode array where a plurality of semiconductor laser diodes, such as 20 semiconductor laser diodes, are arranged with a certain spacing perpendicular to the direction of an optical waveguide to cause resonance in order to generate a high single mode output level. In this case, theoretically more than 20 semiconductor laser diodes can be arranged (see L. J. Mawst et al., "Resonant self-aligned-stripe antiguided diode laser array", Appl. Phys. Lett. 60 (6), pp. 668-670, Feb. 10, 1992).
In the second prior art LD, however, since it is structurally complex, it is difficult to manufacture such LDs at a high manufacturing yield. Additionally, the structure has little tolerance for satisfying the resonance-related requirements and hence it is difficult to manufacture such LDs with an enhanced level of reproducibility.